I see no reason why BOINC shouldn't install by default on the SD Card, some devices have the Local Sorage really low, less than 2 GB, so a SD Card helps a lot, specially because they can be big like 8+ GB and BOINC uses a lot of data, sometimes unnecessarily IMO, for example I participate in Roset@Home and looks like the PC version operates just like the Android version, requiring 938 MB per core, mostly because it copies the main database:
Decompressed on the slots minirosetta_database directory/folder is 483.53 MB (I use Total Commander as a File Manager in Windows), compressed is minirosetta_database_1a38360_n_methyl.zip 293.29 MB so, On the Slot Folder, I compared Slot 0 and slot 10 minirosetta_database folder by content using Total Commander and they're identical, so if it's just a read-only database why even bother decompressing it? just read from the .zip file whatever the project needs and save in my case of a 8-Core CPU 3,884.24+ MB which sometimes is more because more 8+ slots are generated, this would make BOINC project even more portable, easier to run and more devices would be running it, because many devices can't run because space limitation-only, like 2 phones I could install BOINC ont to but can't because of these reasons: mobility and space, mainly mobility, but space is a necessary optimization should occur, only the files that will be modified should be copied to the Slot folders.

SD Cards now a days are reliable and can't be access with full permissions, so not running from it or storing data on them is just unacceptable, LInus Tech Tips YouTube channel just booted WIn 10 64-bit from it, what else you need to make this necessary move? I have apps that can be moved to a SD from long ago, and they never have a problem, Total Commander for Android asks for permission once and it runs without problems, this is just negligent development, the way you're doing it now, so I hope you'll reconsider and correct this flaw for the sake of Scientific Research and betterment of Humanity.

I think would be great if BOINC could use more than 1 SD Card, even the device Local Storage if the SD Cards run out of space, rare but not impossible, but main and default would be the SD Cards

Already in our Issues list at #1599.
Do know we don't at this time have an Android developer, so this won't be done quickly.

I also don't think it should by default install on the SD card, as there are plenty of devices out there that don't have an SD card, and to obligate everyone to first go out and buy one before they can use BOINC is the wrong method of doing so. Two of my Android devices happily run BOINC and neither has an SD card.

Also know that Android doesn't use the 'store up to N days' and 'additionally' preferences, so the cache it does store is only the tasks it's running plus one or more when those tasks are almost done.

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OK, so your situation is even worse than I thought, that's why the Android version is behind the PC version.

By default doesn't mean at all necessarily, means that the first thing to try, of course it will install on the Local Storage if no SD Card is found, I never indicated or implied otherwise.

The cache works like that on Smartphones maybe, but on Tablets, sometimes stores up to 4 or 5 times what it can run at once

Android has already several years, and several versions, must be thousands of developers for it, maybe you should request for experienced ones on your main webpage, maybe even gather donations to pay a normal salary as a non-profit, I'm sure people will donate to BOINC for the great humanitarian work that ti does, like Wikipedia, but your expenses records should be public and available on your website, so you shouldn't be shy to ask if you need some, pretty sure you can gather 60K $us to pay an experienced Android developer 5K $us which I think a normal salary in USA or any developed Country IMO

Cheers

I meant 5K $us/Mo total 60K $us/year