Mac OS X Lion will be throwing away the DVD — and heaven help you if you need it. Apple says the $29.99 operating system update will be available only as a download from the Mac App Store, which itself only runs on 10.6.6 Snow Leopard and later. That leads us to ask a number of questions to which we may not get answers until the day the Mac App Store suddenly says Lion is available for purchase.

Installation -- Apple has told us a fair amount about parts of the installation process. We know that you’ll be able to install Lion on multiple computers, as long as they all share the same Apple ID. In essence then, the $29.99 fee for Lion takes over for the family pack, previously $49.99 for Snow Leopard or $199 for Leopard.

However, we don’t know if or how Apple will restrict the number of installations — presumably there’s a difference between installing one $29.99 copy of Lion on your iMac and MacBook, and installing it on every Mac at your 25-person office. And while such a small office could at least conceive of downloading Lion 25 times, what about a large enterprise installation that might need hundreds or thousands of installations?

One possibility, though an unlikely one, is serialization, where you could download a single copy and then install it multiple times by using appropriate serial numbers. Apple has never required a serial number to install a desktop Mac OS X client before, although Mac OS X Server formerly required a serial number. Lion Server will be a separate $49.99 download from the Mac App Store that will install on top of the regular Lion.

After purchasing and downloading Lion via the Mac App Store, the installer will be available in your Applications folder. Ostensibly, you could then copy it to other computers, and run it on them without downloading again. Lion’s system requirements list an Intel Core 2 Duo or newer processor, which includes machines all the way back to 2006. But can you launch Lion’s installer on a Mac running 10.4 Tiger or 10.5 Leopard? Or will you need to upgrade to Snow Leopard before installing Lion? We’ll have to wait to find out, although Apple’s Web site implies that upgrading to Snow Leopard first will be mandatory. (The developer previews likely can’t answer the question, as they aren’t designed for mass-market installation.)

We’re particularly concerned about the story for people without the bandwidth or with bandwidth limits. Some people have low-bandwidth accounts (satellite-based Internet connections, for instance, or slow DSL connections), and 25 percent of Americans still use dial-up to access the Internet. Plus, in many parts of the world, Internet access is metered, and a multi-gigabyte download could come with non-trivial costs. As far as we can tell now, people in this situation will have to find an alternative connection. But what if they don’t have a laptop or don’t have an alternative nearby? Might Apple make a DVD installation option for Lion available for people for whom a download simply isn’t feasible? Will someone need to haul their 27-inch iMac to the nearest Apple Store or approved retailer? We can hope, but Apple has said nothing on the topic.

Restoration -- Of course, you only have to install Mac OS X once… if you’re lucky. As we all know, installations become corrupted, hard disks fail, people upgrade to larger disks, and so on, all of which can require reinstalling the operating system. In the past, you always had a Mac OS X installation DVD to fall back on, but if you’re getting Lion from the Mac App Store, you’ll have to use alternative recovery methods (for an indication of what they might involve, see “Recovering from Disk Corruption Without a SuperDrive,” 10 June 2011).

You can of course restore a Lion system using a Time Machine backup, as with earlier versions of Mac OS X. But Lion also creates a special recovery partition on your internal hard disk that enables you to boot into a minimal installation of Lion that contains all the standard Mac OS X utilities for repairing another partition or restoring from Time Machine. That’s a good move on Apple’s part.

But what if your hard disk is damaged such that you can’t boot from the recovery partition, and you don’t have a current Time Machine backup, or if you use another backup method that doesn’t create a bootable volume? We’re not sure if you can create a bootable version of the Lion installer on DVD or flash drive, given that what you get from the Mac App Store is itself an app, not a disk image. The solution to that problem will be especially important to Mac consultants, who are regularly faced with needing to bring an ailing Mac back to health no matter how negligent its owner has been in terms of backups or caring for the hardware.

Most people undoubtedly won’t face these difficulties, but not only are these problems critical for those who do encounter them, the increasing popularity of the Mac — 54 million active users, remember — means that the raw number of people who are going have problems installing or reinstalling after something goes wrong will be large.

We’re actively trying to find out what Apple’s answers to these questions are, but the company is not known for being forthright, and we may not have solid information until Lion hits the Mac App Store sometime in July.