I can't use the HD (1T) on a PC anymore after erasing it and backing up. is this reversible or do I have to buy a new HD for PC use only? Could I try erasing the Time Machine archives?
MacBook Air, null
You need to either reformat or repartition the drive, or install a product such as MacDrive on the PC.
(136498)
Is the disk going to also be used for Windows backups? If so, I would lean towards giving Windows its own dedicated backup disk.
Disks are cheap compared to the cost of loosing your unique data.
But if you do want to share that disk with Windows, I would go with Vishal2014's idea and partition the disk with a large partition Mac OS Extended (Journaled) for Time Machine, and an exFAT partition for Windows if you intended to use the disk to transfer data between Mac and Windows, or NTFS if the Windows partition is Windows ONLY and going to be used for Windows backups.
I recently configured an external HD to use with Time Machine and now I can't use it on a PC anymore.
