Read a very disturbing article at LarryJordan.biz, courtesy of The Editblog:

Magnetic signals recorded on a hard disk are designed to be refreshed periodically. If your hard disks stay on, this happens automatically. However, if you store your projects to a removable hard drive, then store that hard drive on a shelf, unattached to a computer, those magnetic signals will fade over time… essentially, evaporating.

Emphasis is mine.  Read his whole article here.

If you are doing any kind of editing with external HDs (I currently have more than 4TBs worth), the article is worth the read.

Then go back and pay a visit to all those HDs.

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1. Destin Log article
2. Juggling Real Life and Writing
3. Future of Entertainment