Following a few weeks of rumor and speculation, Apple today released the Mac OS 8.5.1 Update. The update doesn't offer any new features (although it does include a few new Sherlock plug-ins), but instead addresses a selection of under-the-hood problems in Mac OS 8.5. Although these issues have not affected the majority of Mac OS 8.5 users, Apple recommends that all users of Mac OS 8.5 install the update.
It's always a good idea to perform a complete backup of your system before installing any new system software. This recommendation has nothing to do with Mac OS 8.5.1 - it's just a common-sense practice that can't be over-emphasized.
Update Availability -- The Mac OS 8.5.1 Update is available for free as a 3 MB, self-mounting, disk image from Apple's support sites. This update supports only the North American English version of Mac OS 8.5; Apple says localized versions of the update will be available later this month.
Apple also plans to begin distributing the Mac OS 8.5.1 Update on CD-ROM in January of 1999 (presumably just in time for Macworld Expo in San Francisco). Users in the United States and Canada will be able to order the CD-ROM for about $10 by calling the Apple Software Order Center at 800/293-6617; there's no information yet on the availability of localized versions of the Mac OS 8.5.1 update on CD-ROM.
You can find more information about Mac OS 8.5 and its new features, in a series of TidBITS articles published in October.
What's Fixed -- The Mac OS 8.5.1 Update addresses six primary issues, covered both in the update's ReadMe file and a Tech Info Library article from Apple.
An AppleScript memory leak has been fixed. The PowerPC-native version of AppleScript that debuted with Mac OS 8.5 has a great deal of power, but unfortunately, calls to scripting additions that weren't embedded in "tell" blocks caused the loss of a small amount of memory. The problem isn't the size of the leak but its frequency, which can cause some scripts and (particularly) AppleScript applications like Web server CGIs to fail over a period of time, or require users to restart their machines to reclaim the memory.
Mac OS 8.5.1 fixes a memory problem in the Mac OS file system that could cause a crash. The problem appeared only when applications made numerous asynchronous writes to a disk; FileMaker Pro users may have encountered the problem importing a large number of records from a database.
Mac OS 8.5.1 re-enables many third-party ADB devices like joysticks and copy protection dongles that stopped working under Mac OS 8.5.
The Mac OS 8.5.1 Update comes with an updated version of Sherlock that communicates correctly with proxy servers, enabling Sherlock to better execute Internet searches from behind a firewall. The update also includes a few new Sherlock plug-ins for sites like CNN Interactive, Apple's Macintosh Products Guide, and a selection of search engines and online retailers.
The Mac OS 8.5.1 Update includes Drive Setup 1.6.2, which corrects a rare problem with updating volumes. The problem damages the HFS partition information, preventing the disk from appearing on the desktop. Drive Setup 1.6.2 also makes sure necessary patches are correctly installed on drives after an initialization or update; previous versions of Drive Setup failed to install updates correctly in some cases.
Mac OS 8.5.1 includes Open Transport 2.0.2, which fixes a rare problem some machines had with booting from the Mac OS 8.5 CD-ROM.
The Upshot -- As of this writing, it's too early to tell whether Mac OS 8.5.1 Update introduces new problems or incompatibilities, or includes fixes that aren't detailed here. Further, Apple didn't distribute this update widely to developers before release, so it hasn't been subjected to the wide external testing typical of a major system software release. However, this is not a major system software release: it's just a bug fix intended to address a small set of problems. The patches noted above are well-defined, isolated, and introduce no new features, so users can be confident Mac OS 8.5.1 doesn't include major new problems. I've had no troubles installing the update on three of my systems running Mac OS 8.5, and so far have seen no aberrant behaviors.