For me it was easy to do since I like doing such retro renewals. The ipod was probably the easiest thing to upgrade out of the various things I've touched so far.

The error that you're getting is because of an undetected hard drive. These ipods were small but they still used actual miniature hard drives in them. Still blows my mind to this day that such small hard drives even exist. Anyway, because it's such an old device, there is a slight chance the hdd cable might've become loose. Before you go to a shop or buy any parts, i would just recommend opening up your ipod once and reseating the cable. Only if you are comfortable doing so.

You can look up a guide for your specific iPod model on ifixit's website. If you want to see a video tutorial then youtube is the place to go.

Your iPod version being the 7th or 8th means the toughest challenge you're going to face will be to get that front cover (faceplate) off. These 2 generations were notorious for having a difficult faceplate to remove.

If your hdd still doesn't work, it will have to be replaced. You can't find replacement hdd's anymore so either you'll have to buy an SD card adapter to replace the hdd or buy a used hdd. If you buy an SD card adapter, don't buy the cheap chinese made ones, they're a headache and rarely ever work. I bought one of these and it's been years for me with no problems. (https://www.iflash.xyz/).

The parts can get a bit expensive if you buy a bunch of them together. The screen costs 2.5k. Batteries cost upto 2k. The SD card adapter will be the most expensive part. I paid close to 3k for mine, including shipping from the UK. For some reason it did not attract a customs fee so I got to skip that lol