I came across this when was installing Slackware 15.0 on IBM x3650 M3 (7945) with LSI MegaRAID SAS 2108. All was good until MAKE USB FLASH BOOT menu. At that time here was some SATA link down errors and all filesystems was remounted ro.
I found that problem was caused by "rescan-scsi-bus -l" executed in /usr/lib/setup/SeTkernel.
So the workaround is to reboot machine at this point (I cannot remount rw) and start over but comment out rescan-scsi-bus commands in SeTkernel. And then don't ever run rescan-scsi-bus
Is this a problem with megaraid_sas kernel module?
I had the same on x3550 M5 (5463) and x3650 M5 (5462) by Lenovo with LSI MegaRAID SAS 3108.
