台北老公寓電梯間有兩扇門。每天刷磁扣走主門，習慣了就不會注意旁邊那扇小木門。停電後磁扣系統重置，刷卡只亮紅燈——不是你家的門壞了，是外層的磁扣系統忘了你。旁邊那扇幾乎看不見的側門，停電後才是真正的入口。
現象
停電數小時後重新開機，選了標著作業系統名稱的磁碟選項，畫面立刻跳出禁止符號。
第一直覺：磁碟損毀。合理。禁止符號就是那個意思，而且剛剛停電，還有什麼比「硬碟壞了」更說得通？這台機器跑著自動排程，停機超過十一小時，整批任務全掛著，精神狀態不會很好。
但資料完好無損。SSH 連進去，什麼都在。
分界點
問題不在磁碟，在韌體層。停電清空了 NVRAM，第三方開機引導程式（OpenCore）的 boot entry 因此消失。韌體不再知道「先走引導層、再進磁碟」這條路。
開機選單上看到的「Macintosh HD」是真實存在的磁碟，沒有任何損壞。但通往它的中間層被抹掉了——韌體試著直接跳進磁碟，跳過引導程式，觸發禁止符號。
一個狀態的改變造成了這一切：NVRAM 在斷電後清空，boot entry 歸零，引導層從韌體的記憶裡消失。磁碟沒變，問題出在引導路徑。
容易誤判的原因
禁止符號在 macOS 的語義裡確實指向「找不到可開機的系統」，但那個「找不到」可以有兩種原因：磁碟壞掉、或引導路徑斷掉。兩者的外觀幾乎相同。
更麻煩的是，開機選單初期可能只顯示「Macintosh HD」，沒有 EFI Boot 選項。刷新之前，看起來真的只有一條路——而那條路走不通。這會加深「磁碟有問題」的直覺，因為沒有備選項可以對比。
實際上，那個不起眼的「EFI Boot」選項需要等選單刷新後才出現，或者需要在特定時序下長按 Alt 鍵才能叫出來。
確認方式
解法不是修磁碟，而是找到那個平時幾乎看不見的 EFI Boot 選項，讓引導程式接管，重新建立通往磁碟的路。壓住 Alt（PC 鍵盤的 Alt 等同 Mac 的 Option），等選單出現，選「EFI Boot」，OpenCore 接手，系統正常進入 macOS，一行資料都沒少。
換了入口，全部回來了。
留給未來的話
這台機器的韌體有一個已知限制：Mac mini 2012 搭配 OCLP，偵測到 OpenCore EFI 分區後會固定顯示開機選單，無法透過軟體繞過。bless --setBoot、LauncherOption: Full 搭配 ForceOcWriteFlash、systemsetup -setstartupdisk，全部試過，韌體就是不寫入 Boot0080/BootOrder。這是硬體層面的限制，不是設定問題。
下次停電後看到禁止符號，先問的不是「磁碟有沒有壞」，而是「NVRAM 有沒有被清空、引導層還在不在」。兩個問題的修法差很遠，診斷順序值得記下來。
— 邱柏宇
延伸閱讀
The No-Entry Sign Isn’t a Dead Disk — The Bootloader Forgot
A power outage lasting several hours. Machine comes back up. You pick the disk option labeled with the OS name — and immediately get the no-entry symbol.
First instinct: the disk is dead. The symbol implies it. There was a hard shutdown. The machine had been running automated schedules, offline for over eleven hours with a full queue stalled. “Corrupted disk” is a reasonable read.
Except the data was completely intact. SSH in, everything is there.
Where the Break Actually Was
The disk was fine. The problem was one layer below: NVRAM had been wiped by the power cut, and with it, the boot entry for the third-party bootloader (OpenCore) disappeared. The firmware no longer knew the route — “go through the bootloader first, then hand off to the disk.”
The “Macintosh HD” entry visible in the boot menu was a real, undamaged disk. But the intermediate layer that was supposed to receive control first had been erased from firmware memory. The firmware tried to jump straight to the disk volume, skipped the bootloader, and hit the no-entry symbol.
One state change caused all of it: NVRAM cleared on power loss, boot entry gone, bootloader path erased. The disk did not change.
Why It’s Easy to Misread
The no-entry symbol in macOS means “no bootable system found” — but that condition has two very different causes: a damaged disk, or a broken boot path. The visual output is nearly identical for both.
The complication: early in the boot menu, only “Macintosh HD” appeared. No EFI Boot option visible yet. With no alternative to compare against, the “something is wrong with the disk” read gets reinforced — because it looks like there’s only one path, and it doesn’t work.
The EFI Boot option appears after the menu refreshes, or after holding Alt at the right point in the boot sequence. Before that moment, the menu looks like it’s already showing you everything.
The Confirm
The fix was not disk repair. It was finding the EFI Boot entry — the side door that’s almost never visible in normal operation — and letting the bootloader take over. Hold Alt (PC keyboard Alt = Mac Option), wait for the menu to populate, select EFI Boot, OpenCore takes the handoff, macOS loads normally. No data missing.
Same disk. Different entry point.
Worth Noting Next Time
This machine has a documented hardware constraint: Mac mini 2012 with OCLP always shows the boot selection menu when it detects an OpenCore EFI partition. Multiple workarounds were tested — bless --setBoot, LauncherOption: Full with ForceOcWriteFlash, systemsetup -setstartupdisk — none of them caused the firmware to write Boot0080/BootOrder into NVRAM. The firmware always reasserts the menu. This is a hardware-level constraint, not a configuration gap.
After the next power loss with a no-entry symbol: the first question isn’t “is the disk damaged?” It’s “did NVRAM clear, and is the boot entry still there?” The two diagnoses point to completely different fixes. The order matters.
— 邱柏宇
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