I use a lot of old kit. Recent linux kernels are starting to have problems with old kit. An old laptop harddrive suddenly started to 'freeze' during bootup. The kernel was trying to access it with various DMA modes, none of which seemed to work. Finally it would fall back to PIO, and work, but this was annoying, because it meant the laptop took ages to boot.
I'm attaching a dump of what the kernel spewed out while it was trying to talk to the hard-drive. The main thing to look for if you have the same problem is:
[ 23.107017] ata2: soft resetting link
[ 23.259250] ata2.00: configured for MWDMA1