1541-UII: FW 3.10a causes almost-unrecoverable brick (2.6k downgrade fixed it)

[GerbilSoft]

I attempted to upgrade my 1541 Ultimate II from 3.7 to 3.10a (Dual Disk + ACIA). The updater completed, and after rebooting, the 1541 Ultimate II was essentially bricked. Nothing I tried in #31 helped, including ultimate.bin or recover.u2u (using 3.x). Renaming 3.10a's u2u as recover.u2u sometimes booted (other times just giving me the C64/C128 BASIC prompt); when it booted and I selected "Yes" to flash, it would show 176 flash blocks free and then hang.

The only solution I found was from the old 3.0b7 package. Copying revert.u2u to the SD card and renaming it recover.u2u successfully reverted back to 2.6k, at which point I was able to upgrade to 3.0b7, then 3.1, then 3.10.

The 3.10 firmware seems to be working fine; it's only 3.10a that causes the almost-complete brick.

I'll try 3.10a (Audio) next to see if that makes any difference.

[GerbilSoft]

3.10a (Audio) seems to be working without any issues.

[eightbit0675]

I have the original Ultimate II and I had zero issues upgrading. I went from 2.5 to 3.0b7, to 3.7 and then 3.10a (Dual Disk + ACIA) with no issue.