> ad0: FAILURE - READ-DMA status=51<READ,DSC,ERROR> ERROR=40<UNCORRECTABLE> > LBA=2398527 [...] > panic: vrele: negative refe cnt > cannot dump: no dump device defined It is caused by unmounting file systems with i/o errors. If your disk cannot remap faulty sectors, you should buy new disk. Usually `fsck -y` should help.