I have a Workcentre 7845 that boot loops at the green screen.
I replaced the hard drive and attempted an Altboot recovery, but it ultimately gives a "Software Upgrade Error" page and after pressing "0", it just boot loops at the green screen.
When booting with the old drive, the unit will just click and reboot every 20-30 seconds.
The hard drive was installed with no partitions and the AltBoot thumbdrive must be working, because it starts the software upgrade process. Initially, in that process, the copier will make the initializtion noises like it's powering up, but it'll eventually fail with the "Software Upgrade Error" page.
Before the error page appears, there's a lot of USB activity, like it's copying files, but the hard drive is never partitioned and the USB drive ends up losing its partition.
The LED indicator on the board shows a "u" with the dot flashing for a while, then "u" by itself.