I bet with a Raspberry Pi, an SSD (or an M.2) card, a card receptacle, some jumpers and a little screen, Oh, and some Python, a ten year old could do it. But I ain't ten any more!
I looked into this many years ago, there was a small unit, I can't remember the name, back in 2005? Had a battery pack, a hard drive and Firewire interface, don't think there was a screen.
I got really excited when the iPad came along but the connectivity is so constrained to be practically useless, apart from selfies etc. the level of which holds no interest for me.
I used my Mac Book Pro for several years but the battery has now packed up and I don't see investing in another, (battery or MBP) because I so rarely used it. My original idea was to be able to review my images while on location, giving me the option to re-shoot if any important images were substandard but by the end of the day I am usually so knackered I just drive home regardless, past caring!