Save and retrieving my data

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I often compute things that take a lot of time. When I stop working, I have to turn off my laptop. When I resume my work later, I'd like to be able to retrieve the previous data, computations, etc. For instance, I have computed a big list and stored it to the variable x. I would like to be able to save my data including the contents of this variable, then when I resume my work, I'd like to be able to have x back.

I tried to save my data to a file mydata.dat via the Data -->Upload or create file menu. Then, I quit sage, ran it again, and tried to link the data file back via the same menu. I got the following:

The data filename already exists in other worksheet Delete the file in the other worksheet before creating a link.

I don't really understand what that means. Then, I tried all of the following:

DATA+'mydata.dat'
open(DATA+'mydata.dat')
DATA=DATA+'mydata.dat'

but in any case, if I type x, I get

NameError: name 'x' is not defined

Could somebody tell me how I should achieve what I'm trying?