Check out one of these suppliers:
TiVo Upgrade Kits and Hard Drives
TiVo Upgrade and Repair Services Including TiVo Hard Drive Replacement
I looked up the Philips Series 1 box I have, but if you have another, then you can look it up by model number. If there is an upgrade kit then you're in luck.
Check out this thread:
Drive Expansion and Drive Upgrade FAQ - TiVo Community
I don't remember seeing anyone talking about upgrading a Series 1 themselves, but since there are upgrade kits, perhaps there is a do-it-yourself method.
I hope this helps.
Can I add a hard drive to a Tivo series 1
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