Mac and Linux machines tend to use different formats by default which are not readable by Windows. Windows generally uses NTFS, or variations of the FAT file system, as disk formats. Specifically, the fact that while Windows will recognize hard disks formatted by other Windows machines, hard disks formatted on Mac and Linux machines will appear as unformatted when connected to a PC. There is one software issue that’s worth mentioning before we diagnose hardware, and that’s file systems. More at: Can I Use an Internal Hard Drive from an Old Machine as an External Drive on a New One? File system The technique discussed here - taking an older machine’s hard drive and placing it into an external drive enclosure - is a common and convenient technique, not only for copying data from the old machine but also for continuing to use the drive in a new and perhaps more flexible role.